1. Field of the Invention
This invention relates to an optical scanning apparatus and a projecting apparatus for enlarging and displaying an image based on a light modulating element on a screen surface or the like by a projection optical system, and particularly to an apparatus adapted to display two-dimensional image information by scanning a beam based on image information light-modulated by a light modulating element in which a plurality of pixels are one-dimensionally disposed, in a direction orthogonal to the direction of arrangement of the pixels by light scanning means.
2. Related Background Art
Various projecting apparatuses for enlarging and projecting a film image, two-dimensional image information based on a liquid crystal light valve or the like onto a screen by a projection lens have heretofore been proposed and put into practical use.
FIG. 17 of the accompanying drawings shows the construction of a liquid crystal projector for enlarging and projecting two-dimensional color image information which is comprised of a plurality of liquid crystal panels. A beam from a light source, not shown, is uniformized by illuminating optical systems 102 (102a, 102b, 102c), and liquid crystal panels 103 (103a, 103b, 103c) corresponding to respective color lights are illuminated.
The liquid crystal panels 103a, 103b and 103c correspond to e.g. red, green and blue. Images displayed on the liquid crystal panels 103 are enlarged and projected onto a screen 105 by a projection optical system 104. While in this example, the liquid crystal panels 103 are transmission type liquid crystal panels, use may be made of two-dimensional image display elements such as reflection type liquid crystal panels.
In recent years, it has been required to increase the resolution of an image projected onto a screen surface. To more increase the resolution by a two-dimensional image display element, it is conceivable to increase the number of the pixels of the two-dimensional image display element.
As a method of increasing the number of the pixels of the two-dimensional image display element, there are a method of making each constituent pixel small and arranging a number of pixels, and a method of keeping the size of each pixel unchanged and arranging a number of pixels.
The former method causes the problem that the opening efficiency is reduced because each pixel becomes small. Also, in the latter method, the two-dimensional image display element becomes bulky and therefore, the projection optical system, etc. also become bulky.
In contrast, there is a method of projecting a beam from a one-dimensional image display element onto a screen while scanning it by light scanning means, and forming a two-dimensional image on the screen.
As the one-dimensional image display element, there is an image display element using a diffraction grating, and this is shown on the home page of Silicon Light Machines, Inc., U.S.A. A two-dimensional image display element using a diffraction grating is shown in U.S. Pat. No. 5,311,360, Solid State Sensors and Actuators Workshop, Hilton Head Island, S.C., Jun. 13–16, 1994.
The one-dimensional image display element in the technique shown on the home page of Silicon Light Machines, Inc. is called a one-dimensional grating light value (hereinafter referred to as “GLV”). This GLV comprises a micromachine phase reflection type diffraction grating utilizing the diffraction of light.
When this GLV is utilized, the on-off control of light can be electrically controlled to thereby form image information, and this can be used as a digital image display element.
The construction and operation principle of the GLV will hereinafter be described with reference to FIGS. 14, 15A, 15B, 16A and 16B. FIG. 14 is a perspective view of a GLV, and FIGS. 15A, 15B, 16A and 16B are views for illustrating the operation principle of the GLV.
FIG. 14 shows a GLV showing a pixel. As shown in FIG. 14, the GLV is of a construction in which a frame 15 is disposed on a substrate 14 with a spacer 16 interposed therebetween. A clearance 16a equal to the thickness of the spacer 16 is formed between the upper surface 14a of the substrate 14 and a ribbon 17, and the upper surface 14a and the ribbon 17 are in non-contact with each other.
The thickness of the clearance defined by the spacer 16 and the thickness of the ribbon 17 are both determined by the wavelength of light used, and each of them is formed as λ/4 when the wavelength of the light used is λ.
A number of pixels shown in FIG. 14 are disposed in X direction (one-dimensional direction). Such GLV can be made by the minute semiconductor manufacturing technique. The details of the making method are described in the aforementioned literature.
When comparison is made between the numbers of pixels in a case where a two-dimensional image is obtained by the use of a GLV made into a one-dimensional array and a scanning optical system and a case where a two-dimensional image is obtained by the use of a two-dimensional image display element (liquid crystal panel), the numbers of pixels in the longitudinal direction are the same in both cases, while, in the GLV, at least one pixel is enough in the lateral direction and therefore, the number of pixels becomes small. Therefore, the downsizing of the apparatus can be expected.
As an example, in a high definition television (1920×1080 pixels, HDTV: High Definition Television), the number of pixels of a two-dimensional image display element and the number of pixels of an image display element made into a one-dimensional array are compared with each other. The two-dimensional image display element has about 2,000,000 pixels, and the GLV made into a one-dimensional array has about 1,000 pixels.
That is, when the GLV made into a one-dimensional array is used, it becomes possible to obtain a two-dimensional image by pixels of 1/2000 of those of the two-dimensional image display element.
The operation of the GLV is controlled by the on-off of a voltage applied to between the ribbon 17 and the substrate 14, and FIG. 15A shows the x cross-section of the GLV during the off of the voltage, and FIG. 15B shows the y cross-section of the GLV during the off of the voltage. As shown in FIGS. 15A and 15B, the surface of the GLV during the off of the voltage is in a flat state. FIG. 16A likewise shows the x cross-section of the GLV during the on of the voltage, and FIG. 16B shows the y cross-section of the GLV during the on of the voltage. The reference character 17b designates movable ribbons, and the reference character 17a denotes fixed ribbons.
As shown in FIGS. 15A and 15B, during the off of the voltage of the GLV, the ribbons 17b, like the ribbons 17a, keep a constant distance from the substrate 14, and when in this state, an illuminating beam La enters, the total optical path difference between reflected beams reflected on the alternately provided ribbons 17a and 17b does not occur, but the GLV acts as a plane mirror and hardly diffracts and deflects the illuminating beam but regularly reflects the illuminating beam (it does not include regular reflection that light is deflected).
In FIG. 15B, the ribbons 17b are not lowered by an electrostatic force and the ribbons 17a and the ribbons 17b are in the same state and therefore, the ribbons 17b alone are shown.
On the other hand, as shown in FIGS. 16A and 16B, during the on of the voltage of the GLV, the ribbons 17b are lowered to the substrate 14 side by an electrostatic force, and when here the illuminating beam La enters, the total optical path difference between a beam reflected by the group of ribbons 17a and a beam reflected by the group of ribbons 17b becomes a half wavelength (λ/2). Thus, the GLV acts as a reflection type diffraction grating, and regularly reflected beams (0-order lights) interfere with each other and negate each other, and diffracted lights of other orders (here first-order diffracted lights) are created.
Also, in order to realize such a mechanical operation, the dimension, tensile stress, etc. of the ribbon 17 in the lengthwise direction thereof (Y direction) are determined with the necessary operating speed, restitutive property, etc. taken into account. According to the aforementioned literature, an operating speed of 20 μsec. is obtained when y0 which is the dimension of a diffraction effective area in the lengthwise direction (Y direction) in the ribbon 17 shown in FIG. 14 is 20 μm.
The size of a GLV in Y direction at this time is about 25 μm including the frame 15. Also, the width x0 of the ribbon 17 is determined by the wavelength and the angle of diffraction θd of the illuminating beam, i.e.,dsinθd=mλ.  (1)d is the grating pitch of the diffraction grating, and is determined by the width x0 of the ribbons 17a and 17b, and becomes equal to the pitch of the ribbons 17a or the ribbons 17b. θd is the angle of the reflected beam from the GLV, λ is the wavelength of the illuminating beam, and m is the diffraction order.
An optical system or an apparatus according to the prior art for controlling the propagating state of light by the use of a light modulating element such as a GLV for modulating light chiefly by diffraction, deflection or scattering has left room for improvement in respect of size.